1. Field of the Invention
The present invention relates to an image pickup system for imaging an object to form images, an image adjustment system, a color chart for chroma control of the image pickup system, and an image adjustment method for adjusting the images.
2. Description of the Related Art
Hitherto, in imaging devices such as electronic cameras, in order to correct characteristics such as the spectral transmission of an imaging optical system or the spectral characteristics of a color filter of an imaging element, initial adjustment of characteristics for white balance and color balance is employed, for example, at the factory shipment or at a maintenance division. For example, by using a light box or a color-bar chart as a reference light source, the reference value for white balance and a linear matrix may be adjusted.
A conventional image pickup system is described below with reference to the drawings.
FIG. 10 is a block diagram of an imaging system of a conventional electronic camera. In FIG. 10, there are shown a light box/color-bar chart 300 used for adjusting color balance, a lens 301, a diaphragm 302 for adjusting light exposure, an imaging element 303, an analog front end 304 for A/D conversion by sampling the output of the imaging element 303, a signal processor 305, an image output unit 306, a white-balance adjustment unit 307 included in the signal processor 305, a color-balance adjustment unit 308 included in the signal processor 305, a photometric unit 309, an exposure control unit 310, a diaphragm drive unit 311, a photometric region A 312 for providing a region where the photometric measurement is performed with the photometric unit 309, a color-adjustment control unit 314, a mode switching unit 315 for switching the mode between an imaging mode and an adjustment mode, and a white chart 320 for use in white balance adjustment.
In an imaging mode, a light ray incident on the lens 301 is photo-electrically converted by the imaging element 303 after receiving appropriate exposure adjustment with the diaphragm 302. The image signal is then sampled and A/D converted in the analog front end 304 so as to produce an output from the image output unit 306 after being processed by the signal processor 305. At this time, in the photometric unit 309, the A/D converted image signal is sampled over a region defined by the photometric region A (312) so as to control an exposure amount in the exposure control unit 310 based on the photometric result while the state of the diaphragm 302 is controlled by the diaphragm drive unit 311. In general, for photometry over the photometric region A (312), center-weighted average photometry is used, so as to have the photometry centering a central object as well as taking the vicinity into consideration. In the imaging mode, the white balance adjustment unit 307 and the color balance adjustment unit 308 have been pre-adjusted and process a color signal based on the pre-adjustment so that an optimal color signal is produced.
FIG. 11 is a flowchart showing a color adjustment process of a conventional imaging system. FIG. 12 is a characteristic diagram showing luminance levels when a white chart is imaged. FIG. 13 is a characteristic diagram showing luminance levels when a 7-color color-bar chart is imaged in the conventional imaging system. Signal processing in the imaging system of FIG. 10 will be described below with reference to the flowchart of FIG. 11.
To employ color adjustment, white balance adjustment is initially performed.
Specifically, the image of the white chart 320 (FIG. 10) is captured (Step S1101). Subsequently, a field angle is aligned (Step S1102). Then, the light ray from the white chart 320 and through the lens 301 is photo-electrically converted by the imaging element 303 after receiving appropriate exposure adjustment with the diaphragm 302, and is sampled and A/D converted in the analog front end 304. In the photometric unit 309, the A/D converted image signal is sampled over the region defined by the photometric region A so as to perform center-weighted average photometry (Step S1103). Subsequently, on the basis of the photometric result, the amount of exposure is controlled in the exposure control unit 310 (Step S1104), and the state of the diaphragm 302 is controlled by the diaphragm drive unit 311 (Step S1105). The luminance level herein of the exposure-controlled image signal becomes 50% as shown in FIG. 12, so that the image signal of the white chart controlled at 50% is entered to the signal processor 305.
In Step 1106, white balance adjustment is carried out in a color adjustment mode (selected using the mode switching unit 315). White balance adjustment is implemented by using the color-adjustment control unit 314 to control the white-balance adjustment unit 307. The white balance adjustment for white balance control herein means the production of information, such as the control range of the white balance control, by sampling a reference image of a color temperature (white balance control attempts to control according to variation in environmental and lighting conditions when an object is imaged, by selecting color temperature range, which is pre-adjusted at white balance initial adjustment of the factory shipment or a maintenance division, corresponding to various lighting conditions). Here, the adjusted control range to be used for the white balance control is obtained by sampling reference images of color temperatures.
Next, the color balance is adjusted.
Specifically, the chart to be imaged is replaced with the 7-color color-bar chart 300 (FIG. 10) at first (Step S1107). Subsequently, the 7-color color-bar chart 300 is imaged (Step S1108), and a field angle is aligned (Step S1109). Then, the light ray incident on the lens 301 by imaging the 7-color color-bar chart 300 is photo-electrically converted by the imaging element 303 after receiving appropriate exposure adjustment with the diaphragm 302, and is sampled and A/D converted in the analog front end 304. In the photometric unit 309, the A/D converted image signal is sampled over the region defined by the photometric region A so as to perform the center-weighted average photometry (Step S1110). Subsequently, on the basis of the photometric result, the amount of exposure is controlled in the exposure control unit 310 (Step S1111), and the state of the diaphragm 302 is controlled by the diaphragm drive unit 311 (Step S1112). The luminance level herein of the exposure-controlled image signal becomes 50% in entire average as shown in FIG. 13, so that the image signal of the 7-color color-bar chart controlled at 50% is entered to the signal processor 305.
Note that the field angle during imaging of the chart should be strictly aligned. The reason is that because of the center-weighted average photometry, if the peripheral region of the chart is reflected, or the sides of the chart are cut off, the average luminance level is changed, so that an appropriate amount of exposure cannot be obtained. This is the same in the case of a white chart, so that if the field angle alignment is not appropriate, the signal level is changed and the precise adjustment cannot be obtained.
When the color adjustment mode is selected by the mode switching unit 315, the color balance is adjusted by the color-adjustment control unit 314 in the color-balance adjustment unit 308 (Step S1113). The color balance herein means the adjustment of color tone and chroma of each color. Color balance adjustment can be specific to certain colors, for example, yellow and red as will be described below.
Color balance is generally adjusted with respect to color-difference signals Cr and Cb by changing gain and color tone. This is principally equivalent to that even when adjusting by multiplying the RGB by a matrix factor. Hence, when the matrix is linear, three or more kinds of color tones cannot be independently adjusted. In this regard, proper reproduction of the color of the human flesh (also a memory color) is desirable. Since yellow and red do impact the color of the human flesh, it is desirable to adjust both colors to obtain desired target values. Accordingly, the color balance adjustment for yellow and red is implemented by sampling yellow and red portions of a 7-color color-bar chart so as to determine a matrix factor so that the colors have desired target values.
However, in the conventional imaging system, when images produced by imaging an object are adjusted in color, the white chart for adjusting the white balance and the color-bar chart for adjusting the color balance has to be replaced, thus, unnecessarily increasing the adjustment time for adjusting white balance and color balance. Moreover, since in the photometric region for color adjustment, the center-weighted average photometry is used in the same way as in the photometric region for the imaging, minute dispersion in field angle affects exposure, which can cause inaccurate color adjustment.